Musannaf Ibn Abi Shayba 13:2341

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from al-A'mash, from Zayd ibn Wahb, who said: Umar said: "If you pass by a camel herder, then call out, 'O herder!' three times. If he answers you, then ask him for water to drink. If he does not answer you, then go to them (the camels), milk them, and drink, then tie their teats."